Formation mechanism and optimization of highly luminescent N-doped graphene quantum dots

Abstract

Photoluminescent graphene quantum dots (GQDs) have received enormous attention because of their unique chemical, electronic and optical properties. Here a series of GQDs were synthesized under hydrothermal processes in order to investigate the formation process and optical properties of N-doped GQDs. Citric acid (CA) was used as a carbon precursor and self-assembled into sheet structure in a basic condition and formed N-free GQD graphite framework through intermolecular dehydrolysis reaction. N-doped GQDs were prepared using a series of N-containing bases such as urea. Detailed structural and property studies demonstrated the formation mechanism of N-doped GQDs for tunable optical emissions. Hydrothermal conditions promote formation of amide between –NH2 and –COOH with the presence of amine in the reaction. The intramoleculur dehydrolysis between neighbour amide and COOH groups led to formation of pyrrolic N in the graphene framework. Further, the pyrrolic N transformed to graphite N under hydrothermal conditions. N-doping results in a great improvement of PL quantum yield (QY) of GQDs. By optimized reaction conditions, the highest PL QY (94%) of N-doped GQDs was obtained using CA as a carbon source and ethylene diamine as a N source. The obtained N-doped GQDs exhibit an excitation-independent blue emission with single exponential lifetime decay.

Tuning the Photoluminescence of Graphene Quantum Dots by Fluorination

“…Aqueous solutions of GQDs and F-GQDs reveal black and achromatic colors, respectively, under sunlight [25]; Figure 3(a) (inset) shows that GQDs and F-GQDs emit bright yellow and green photoluminescence under irradiation by a 365 nm UV lamp, respectively. The GQDs have a weak absorption at 342 nm, which is close to previous reports [26], and a strong broad absorption at 480 nm, which is in the visible light range. F-GQDs have a typical absorption peak at 337 nm corresponding to the → * transition of the C=O bond.…”

Fluorescently tuned nitrogen-doped carbon dots from carbon source with different content of carboxyl groups

“…For instance, Xu et al 28 developed a green heterogeneous synthetic method for NCDs with yellowish-green fluorescence. Qu et al 29 synthesized highly fluorescent blue-emission NCDs with citric acid (CA) and ethylene diamine. In the Qu's work, 30 NCDs prepared with low urea/citric acid mass ratio of 0.2:1 exhibit blue emission, while show green emission when the mass ratio increases to 2:1.…”

Hydrothermal Synthesis of Photoluminescent Nanocarbon from Hydroxylic Acids and Amines

“…In Figure 3, the absorption spectra of the CDs from CA are shown. The absorption band in the UV region at ~340 nm assigned to the n-π* transition [14,22] sharply increases with the reaction time due to the formation of CDs. The contour plots of PL intensities of the CDs from CA with EDA are shown in Figure 4.…”

“…A pair of citric acid (CA) and ethylendiamine (EDA) has been used as the reactants for the pyrolytic and hydrothermal syntheses [14][15][16][17][18], where CA and EDA are the carbon and nitrogen sources. It is noteworthy that an extremely high quantum yield of 0.94 is achieved with the pyrolysis of CA and EDA in autoclave [14]. It is also found that the molecular structures of amines drastically changes the PL properties of the CDs; EDA gives substantially higher quantum yields than other amines such as diethyl-and triethylamines [16].…”